Generally, an optical device used in the optical communications systems has two polarization principal axes (PPAs) orthogonal to each other. The difference in loss between the two PPA is referred to as polarization dependent loss (PDL).
FIG. 1 is a schematic diagram of an optical device 1 having a PDL. The optical device 1 includes polarization beam splitters (PBSs) 2, 3 and optical waveguides connecting the PBSs 2, 3. The PBS 2 splits input light into a TE (Transverse Electric) wave and a TM (Transverse Magnetic) wave, those of which travel along the two orthogonal polarization principal axes, respectively. The PBS 3 combines the TM wave and TE wave and outputs the combined waves. A loss 4 is added to only the TE wave in the optical waveguide connecting the PBSs 2, 3.
Note that many actual optical devices do not always include the PBSs 2, 3 and loss 4 as explicit components and the optical device schematically shown in FIG. 1 is just an example to explain the PDL. Also note that a loss is added to not only the TE wave, but also to the TM wave; however, the loss 4 described herein denotes the difference in loss between the TE wave and TM wave. When the loss of the TM wave is larger than the loss of the TE wave, the loss 4 is to be added to the TM wave. In the case of a gain device, the difference in gain between the TE wave and TM wave is referred to as a polarization dependent gain (PDG). The PDG and PDL have a common in that both introduce differences in intensity between the two polarization principal axes, and therefore both the PDG and PDL are collectively treated as PDL in this description.
Upon input of an optical signal to an optical device with a PDL, insertion loss of the optical device varies according to the polarization states of the optical signal. As a result, the optical communications system may sometimes experience deterioration in transmission quality which in turn contributes to network failure. In the case of an optical polarization multiplexing transmitter, which combines two optical signals whose polarization states are orthogonal to each other and outputs the combined signals, the difference in loss introduced to the two optical signals before being combined becomes PDL.
FIG. 2 is a schematic diagram of an optical polarization multiplexing transmitter 5 having a PDL. Light output from a laser light source (Laser Diode: LD) 6 is split by a PBS 7 into a TE wave and a TM wave which are orthogonal to each other. After these waves are modulated by optical modulators 9, 10, respectively, a loss 11 is added to only the TE wave. A PBS 8 combines the TM wave and the TE wave and outputs the combined waves. The loss 11 denotes the difference in loss between the TE wave and TM wave. If the loss of the TM wave is larger than the loss of the TE wave, the loss 11 will be added to the TM wave. It cannot be denied that the loss 11 is added not only in the optical waveguide, but also in the PBSs 7, 8, the optical modulators 9, 10 and other components.
The PDL of the optical polarization multiplexing transmitter 5 increases an intensity difference between the two polarization waves before being combined and output. Consequently, the optical signal with the power-reduced polarization wave decreases the tolerance to light noise (i.e., amplified spontaneous emission noise), thereby drastically deteriorating the transmission quality.
FIG. 3 is a graph showing degradation of receiving sensitivity of a coherent receiver that receives dual polarization-quadrature phase shift keying (DP-QPSK) signal light generated by an optical polarization multiplexing transmitter 5. As shown in FIG. 3, for example, the tolerance to light noise decreases by as much as 1 dB or more for a PDL of 2 dB.
The PDL can be compensated for by another optical device in which its two polarization principal axes with losses are replaced with each other. This optical device is called a PDL compensation device or the like.
However, which of the waves traveling along the two polarization principal axes has a larger loss in an optical device having a PDL (hereinafter, the polarization principal axis having a larger loss is expressed as “an orientation of the PDL”) varies from device to device. Even if the orientation of the PDL of an optical device is fixed and if the optical device with the PDL is connected with a PDL compensation device with a regular optical fiber, the polarization waves of an optical signal freely rotate in the optical fiber and the polarization state of the optical signal cannot be stable in the PDL compensation device, resulting in inappropriate compensation for the PDL.
Japanese Unexamined Patent Application Publication No. 2003-14956 discloses the background art of the present technical field. The publication describes a structure in which two fiber bragg gratings (FBGs), each having the same PDL, are connected with a polarization maintaining fiber (PMF) twisted by 90°. The PMF is an optical fiber applied with stress so as to constantly rotate the polarization of optical signals and is capable of obtaining optical signals whose polarization state stays the same at input and output terminals. The PMF twisted by 90° connects the two FBGs so that their PDLs are oriented orthogonal to each other, and the PDLs of the FPGs are thereby compensated for.
Japanese Unexamined Patent Application Publication No. 2004-151671 discloses a structure in which two Mach-zhender optical waveguides, each having the same PDL, are connected with a PMF twisted by 90°.